In order to increase the operating temperature and thereby the efficiency of gas turbine engines, various grades of silicon nitride are being investigated as potential materials for the hot flow path components of such engines. These various grades of silicon nitride fall into two general categories: hot pressed silicon nitride (HPSN) and reaction sintered silicon nitride (RSSN). From a property standpoint HPSN is a fully dense material and offers the highest strength whereas RSSN, being porous, has a lower strength than the HPSN. However, from the fabrication standpoint RSSN is preferred because it can be formed by injection molding techniques, slip casting techniques, and other techniques into complex shapes, while HPSN is formed by compacting the powder at high temperatures and is limited to simple shapes. Machining of HPSN to achieve a complex shape is not an attractive approach because it requires diamond grinding which is extremely costly and the shapes which can be made are still limited.
A method of combining the advantages of both classes of materials has been disclosed previously for manufacturing ceramic rotors for gas turbine engines. In this approach, a complex blade ring is made from RSSN and the hub is made from HPSN. The two parts are then diffusion bonded together in a separate operation. See U.S. Pat. No. 3,854,189. On the other hand, the HPSN may be hot pressed and simultaneously diffusion bonded to the RSSN material in a single operation. This is the preferred way of doing so and it accomplishes a bond of better quality.
This specification teaches an alternate method for achieving a high density silicon nitride article of complex shape. The method taught herein is one in which articles having complex shapes are formed originally as an RSSN material and then subsequently are subjected to pressures which densify the material to that greater than originally achieved. One may even reach a theoretical density for silicon nitride material by our process.
It is a principal object of this invention to provide a method for making complex articles of reaction sintered silicon nitride and then subsequently densifying such articles so that they have strength characteristics approaching hot pressed silicon nitride.